1. Field of the Invention
The present invention relates generally to processing and display of seismic data, and more particularly to processing and display of seismic data using a modified Hilbert transform.
2. Description of Related Art
To find the most likely locations of hydrocarbon deposits beneath the earth's surface, geophysicists use techniques that evaluate and interpret the structure of subsurface formations. The effects of stratigraphy, lithology and pore fluid content on geophysical data, properly interpreted, can be evaluated to predict the likelihood that hydrocarbon deposits are present in the volume under analysis.
Traditionally, subsurface formations are analyzed using seismic reflection data that is processed to image acoustic boundaries in the subsurface. Seismic wave energy projected into the earth reflects in particular ways from subsurface changes in the rock properties or from combinations of such changes. The reflected seismic wave energy can be recorded at a number of shotpoints, resulting in similar artifacts on a number of adjacent seismic traces. These are interpreted as a continuous event in the reflected seismic data.
Various methods have been disclosed for detecting and displaying abrupt changes in subsurface formations within a given three-dimensional subsurface volume based on such seismic data. The goal of processing the data is to provide a detailed visual display that accurately represents the faults, channels and other geophysical features in the subsurface volume under consideration. Typically, the properties of the subsurface volume are visualized using a powerful computer such as a high end graphics workstation that can generate three-dimensional displays and/or printouts of the geophysical properties of the volume based on the collected seismic data.
A number of techniques have been disclosed that employ, in various ways, data relating to adjacent seismic traces for detecting abrupt geological changes in a given three-dimensional subsurface volume, where each seismic trace corresponds to a particular location on a subterranean surface and is produced by detection of seismic waves by receivers having that location as their common depth point. For example, U.S. Pat. Nos. 5,563,949 and 5,838,654 describe methods that are based upon the coherence of adjacent seismic traces. U.S. Pat. No. 5,724,309 discloses a method that is based on the calculation of spatial derivatives of adjacent seismic instantaneous phases. U.S. Pat. No. 5,831,935 discloses yet another type of method of processing seismic data based upon subtraction of adjacent seismic traces. And U.S. Pat. No. 5,892,732 discloses a method that includes the computation of eigenvalues derived from adjacent seismic traces.
Another type of conventional method uses a Hilbert transform to process the seismic data. For example, U.S. Pat. No. 4,945,519 discloses a method in which individual traces are processed by exponentiation to a power greater than one, the Hilbert transform of the processed traces is determined, and then ratios of the transformed traces are compared. Another example is found in U.S. Pat. No. 4,633,399, wherein the seismic trace data is rectified, Hilbert transformed and then determined as an arctangent function that constitutes a new phase parameter. However, methods applying Hilbert transforms to other than three-dimensional subsurface volumes are not particularly relevant to generating from seismic data an image representing the volume in three dimensions.
Some of the above methods, and others known in the art, have provided improved displays in their own contexts. However, in many instances these methods result in depictions of structural features such as faults, and stratigraphic features such as facies changes from reservoir quality rock to non-reservoir quality rock, that either lack clarity or simply do not appear in the depiction. As a result, even a highly trained interpreter may be unable to identify or predict the likely petrophysical characteristics of the three-dimensional subsurface volume under examination.
More specifically, the inadequacy of conventional methods for processing and displaying seismic data shows up particularly in the difficulty they encounter in clearly highlighting possible faults and channels associated with gradual, as compared with rapid or abrupt, changes in the reflected seismic wave fields. This limitation is due, at least in part, to the fact that currently available detection methods were designed to detect only abrupt local changes in the seismic wave fields, and do not address the detection of gradual changes.